A known single mode optical fiber component is the optical fiber Bragg distributed grating. Such a component is useful as an optical filter which can readily be made in a form that is both relatively highly wavelength selective and polarization state insensitive.
One method by which such a grating can be made is described in GB 2 161 612A, and involves launching a high-power beam of light into a length of photo-responsive single mode fiber and using a reflector to set up a standing-wave pattern in the fiber. This standing-wave pattern produces localized changes in the refractive index of the fiber resulting in the production of a barrow stop-band filter that is selectively reflective at the wavelength of light employed to make the grating. Another method by which such a grating can be written is described by G Meltz et al in article entitled: "Formation of Bragg Gratings in Optical Fibers by Transverse Holographic Method", Optics Letters, 1989, 14, (15), pp 823-825. This involves illuminating the fiber from the side with a holographically generated grating fringe pattern. When such fringe patterns are generated with interfering beams of collimated light, the resulting grating is of uniform pitch, though chirped gratings can also be generated by the expedient of using interfering beams of differing divergence.
Both types of method of making distributed Bragg gratings so far described have involved the use of light to create a phase grating in the fiber. A different type of grating, known as a type II grating, is similarly made in single mode optical fiber by lateral holographic illumination, but in this instance the grating fringe pattern is formed much more rapidly, typically using a single short duration pulse from an excimer laser. The creation of such type II gratings is for instance described by C. G. Askins et al in an article entitled: "Fiber Bragg Reflectors Prepared by a Single Excimer Pulse", Optics letters, 1992, 17 (15) pp 833-835.
Another known single mode optical fiber component is the 2.times.2 single mode optical fiber fused tapered coupler. This may be produced in a highly reproducible manner by a progressive stretching method substantially as described in GB 2 150 703A. The essence of this progressive stretching method of making such a coupler is that two substantially identical optical fibers are stranded together and mounted between a pair of clamps that themselves are mounted on independent motor-driven linearly sliding carriages. The two sliding carriages move along a common axis, and the stranded fibers are arranged to extend parallel to this axis. Movement of the two carriages in the same direction, but with the leading carriage constrained to move slightly faster than the trailing carriage, causes the fibers to be progressively stretched. A relatively sharply localized hot zone, provided for instance by a methane oxygen flame issuing from the end of a length of hypodermic tubing, is moved into position where it locally heat-softens the fibers so that their stretching is accommodated by plastic flow confined to the region of the hot zone. The traversing of the two carriages means that the localized plastic flow is itself traversed along the fibers at a controlled rate. In this way a single traverse of the carriages will produce a drawn-down region of the fibers, the length of which is determined by the extent of the traverse. The cross-sectional reduction is independent of the length of the traverse, and is determined by the ratio of the speeds at which the two carriages are driven. To make a fused fiber coupler by this progressive stretching method, several, or even a few tens of, traverses may be employed to achieve the requisite aggregate drawdown ratio.